ALARA Planning: Time, Distance, and Exposure Control

Stay Time Calculations

The fundamental stay-time calculation is: Maximum time = Dose limit / Dose rate. If the dose rate is 50 mR/hr and your administrative dose limit for the job is 100 mR, the maximum stay time is 100/50 = 2 hours.

The "dose limit" in this calculation is not necessarily the annual regulatory limit. It should be the dose allocation for the specific task, set by the RSO as part of the ALARA review. The annual regulatory limit for occupational dose to the whole body under 10 CFR 20.1201 is 5 rem (50 mSv) total effective dose equivalent (TEDE). Most facilities set administrative limits well below this: 500 mR per quarter or 2 rem per year are common ALARA-driven administrative limits.

For a specific job, the RSO allocates a dose budget based on the expected dose rate, the time required to complete the work, and the worker's accumulated dose for the monitoring period. If a worker has already received 300 mR this quarter and the quarterly administrative limit is 500 mR, only 200 mR is available for the next job. At 100 mR/hr, the maximum stay time for that worker is 2 hours.

When the dose rate varies across the work area, use a weighted-average approach or calculate the dose for each zone separately. If a worker spends 30 minutes in a 20 mR/hr zone and 15 minutes in a 100 mR/hr zone, the total dose is (0.5 × 20) + (0.25 × 100) = 10 + 25 = 35 mR. This is the approach used in pre-job ALARA planning and on radiation work permits.

10 CFR 20 Dose Limits

The NRC dose limits in 10 CFR 20.1201 define the maximum permissible occupational dose. These are legal ceilings, not targets. An effective ALARA program keeps actual doses far below these limits.

• Total Effective Dose Equivalent (TEDE): 5 rem (50 mSv) per year. This is the whole-body limit and the one most commonly relevant for industrial radiation workers.
• Organ or tissue dose equivalent (excluding lens of eye): 50 rem (500 mSv) per year.
• Lens of eye dose equivalent: 15 rem (150 mSv) per year.
• Shallow dose equivalent (skin or extremity): 50 rem (500 mSv) per year.
• Embryo/fetus of declared pregnant worker: 0.5 rem (5 mSv) over the entire pregnancy, with an effort to keep the dose relatively uniform over time.

For members of the public (non-radiation workers), the limit under 10 CFR 20.1301 is 100 mrem (1 mSv) per year, and 2 mR/hr in any unrestricted area. This 2 mR/hr limit is the basis for radiography boundary calculations.

The 5 rem annual occupational limit corresponds to about 2.4 mR/hr for a 2,000-hour work year. In practice, most industrial radiation workers receive annual doses of 0.5 to 2 rem. Exceeding 2 rem triggers increased scrutiny in most ALARA programs. Exceeding 5 rem is a regulatory violation that requires reporting to the NRC.

ALARA Planning for Routine and Non-Routine Work

ALARA is the practice of keeping radiation exposures as low as reasonably achievable, considering economic and practical factors. It is codified in 10 CFR 20.1101(b) and is a regulatory requirement, not a suggestion.

Routine work (daily gauge readings, routine source leak tests, material handling in storage areas) is covered by standing ALARA procedures. The RSO establishes dose rate limits for work areas, requires dosimetry, and monitors trends. If a worker's dose begins trending upward, the RSO investigates and takes corrective action.

Non-routine work (source exchanges, maintenance in radiation areas, emergency response, decommissioning) requires a specific ALARA review before the work begins. The review should include: a dose rate survey or estimate for the work area, the expected duration of each task, the calculated dose for each worker, the specific measures to reduce dose (time reduction, distance, shielding, tooling), and the administrative dose limit for the job.

Three practical strategies reduce dose in nearly every situation:

1. Time reduction: Practice the job on a mock-up before entering the radiation field. Pre-stage all tools and materials. Assign specific tasks to specific workers so nobody is standing in the field waiting for instructions.
2. Distance maximization: Use long-handled tools, remote-operated equipment, or extension rods to increase distance from the source during hands-on work. Even an extra foot of distance from a point source reduces the dose rate significantly.
3. Shielding: Place temporary shielding between the worker and the source. Lead blankets draped over a source container while performing maintenance nearby can reduce the dose rate by 50% or more.

Radiation Work Permits

A Radiation Work Permit (RWP) is a written authorization that specifies the conditions under which radiation work may be performed. While not explicitly required by NRC regulations for all licensees, RWPs are considered a best practice and are required by many Agreement State regulations and facility-specific license conditions.

A well-constructed RWP includes: the work description, the expected dose rates in the work area, the estimated dose to each worker, the dose limit for the job (administrative limit set by the RSO), required protective measures (dosimetry type, shielding, special tools, PPE), survey requirements (pre-job, during work, and post-job), and the RSO's approval signature.

For industrial radiography under 10 CFR 34, the daily job log serves a similar function. The radiographer records the date, location, isotope, activity, exposure time, and any unusual conditions. The RSO reviews these logs as part of the ALARA program.

RWPs are particularly important for non-routine work where workers may not be familiar with the radiation environment. The RWP provides a briefing document that ensures every worker knows the expected dose rate, the time limit, the dose budget, and the specific protective measures before entering the area. A pre-job briefing using the RWP as a checklist is the standard method for communicating this information.

Dose Tracking and Trending

Occupational dose monitoring is required by 10 CFR 20.1502 for workers who are likely to receive more than 10% of the applicable dose limit (500 mrem/yr TEDE for adults). Most industrial radiation workers wear thermoluminescent dosimeters (TLDs) or optically stimulated luminescence dosimeters (OSLDs) processed by an accredited (NVLAP) dosimetry service on a monthly or quarterly exchange cycle.

In addition to passive dosimeters, many workers carry alarming personal dosimeters (APDs, also called electronic dosimeters) that provide real-time dose and dose rate readings. APDs are not the legal record (the TLD/OSLD is), but they provide immediate feedback that helps workers manage their exposure during a job.

Dose trending means tracking each worker's dose over time, looking for patterns. A radiographer whose quarterly dose is consistently 200 mR for six quarters and then jumps to 600 mR in one quarter warrants investigation. Possible causes include a higher-activity source, more overtime, a change in technique, or a dosimetry error. The RSO should review the job logs, compare with coworkers doing similar work, and determine whether the increase reflects a real change in exposure conditions.

Facility dose reports should include: individual doses by monitoring period, cumulative annual dose, comparison to administrative limits, trend charts, and any investigation results for doses that exceeded action levels. NRC Regulatory Guide 8.7 provides guidance on occupational dose recordkeeping, and NRC Regulatory Guide 8.29 discusses risks from occupational radiation exposure.

Spreading Dose Across Workers and Shifts

One ALARA strategy is to distribute the dose among multiple workers rather than concentrating it in one person. If a job requires 4 hours in a 50 mR/hr field (200 mR total), sending two workers for 2 hours each gives each worker 100 mR instead of one worker getting the full 200 mR. This approach reduces individual dose but does not reduce collective dose (the sum of all individual doses, in person-rem).

Whether dose spreading is appropriate depends on the situation. If the job can be done efficiently by one skilled worker, adding a second worker who is less experienced may increase the total time (and collective dose) while reducing the individual dose. If the job is straightforward and can be easily divided, dose spreading is effective. The RSO must weigh individual dose reduction against collective dose and practicality.

For industrial radiography, the crew typically consists of two people (as required by 10 CFR 34.41 for all field radiography operations). Dose is naturally shared because both workers participate in setting up and breaking down the shot. If one worker's dose is trending higher, the RSO may rotate crew assignments so the higher-dose worker takes the assistant role (farther from the source during setup) on subsequent jobs.

Administrative dose limits set by the RSO below the 10 CFR 20 limits enable this management flexibility. If the administrative limit is 1 rem/quarter and a worker reaches 800 mR, the RSO can reassign that worker to lower-dose tasks for the remainder of the quarter while other workers complete the higher-dose work.

Emergency Dose Guidelines

Planned special exposures and emergency exposures are addressed separately from routine occupational limits. Under 10 CFR 20.1206, a planned special exposure allows a dose up to 5 rem TEDE in a single event (in addition to the annual limit), with lifetime tracking and prior NRC authorization. This provision is rarely used in industrial settings.

For emergency response involving a stuck source, overexposure, or lost source recovery, the NRC and EPA have published emergency dose guidelines. The general principle is that the dose accepted during an emergency should be commensurate with the benefit of the action. Saving a life may justify higher doses than recovering a source that is not posing an immediate public hazard.

The EPA Protective Action Guides (PAGs) and NCRP Report 116 provide dose guidelines for emergency responders. For perspective, a dose of 25 rem is the threshold for observable clinical effects (mild blood count changes). A dose of 100 rem can cause acute radiation syndrome. Industrial emergencies involving sealed sources rarely result in doses this high, but the RSO and emergency response team should know these thresholds and have a plan for dose management during a source recovery.

Key point: the best emergency dose management is prevention. Proper maintenance of exposure devices, adherence to operating procedures, and thorough pre-job planning prevent the events that lead to emergency exposures. The NRC's operating experience notifications and information notices document real incidents that provide lessons for preventing similar events.